Former Minister Mentor Lee Kuan Yew is concerned for the many Chinese Singaporean parents who speak English to their children at home, at the expense of Mandarin.
At a dialogue on Friday with 4,000 ethnic Chinese entrepreneurs, Lee said, "I think it's a mistake. They will learn English in school anyway and they should keep up their Mandarin at home so that the child will naturally speak Mandarin and find it easy in school."
At home, Lee prefers to speak to his grandchildren in Mandarin as it ensures that they are conversant in the language which is becoming increasingly important.
He is concerned that some 30 years after English was officially made the first language, many are losing touch with speaking Mandarin.
Lee said that Chinese Singaporeans who gave up Chinese were losing "something very valuable", and would also not be able to take advantage of a rising China and its attractive market of 1.3 billion people.
The 88-year-old also disclosed he has written a book about Singapore's bilingual policy and his experience in learning two languages.
"I spent a whole lifetime catching up with Chinese," he said, adding that Mandarin is his fourth language, after English, Malay and Hokkien.
Through the book, soon to be launched on 28 November, he hopes that the present generation would know how strongly the Chinese community had felt about the language.
"They refused to have Chinese become the second language... They wanted Chinese to be on equal terms as a first language with English, which was not possible," he said.
Mr Lee explained that Singapore is a multi-racial society and Chinese could not be the common language for Singaporeans of all races.
In addition, Singapore needs English to "link up with the rest of the world". Lee reasoned that if English had not been made the first language, "our economy will not flourish, because in the rest of the world, either the first or second language is English".
"China cannot give us the economy that we now have. Even a growing China cannot give us this economy. It's the English language which gives us this access to the world," he said.